Fuel cell gas separator

ABSTRACT

A fuel cell gas separator ( 14 ) between two planar solid oxide fuel cells ( 12 ) comprises a first layer ( 22 ) which is formed of a material that is impermeable to gases, a second layer ( 24 ) which is formed of a material that is impermeable to gases. The first and second layers have perforations ( 28 ) through their thickness which are closed by electrically conductive plug material ( 30 ). A third intermediate layer ( 26 ) between the first and second layers is electrically conductive and is in electrical contact with the plug material in the perforations through the first and second layers. The perforations in the first layer may be offset relative to the perforations in the second layer. The electrically conductive plug material in the perforations of the first and second layers may be the same, and may also be the same as the material of the third intermediate layer. The electrically conductive material may be silver or a silver-based material such as a silver-glass composite. Electrically conductive coatings may be provided over the electrode-contacting zones of the first and second layers.

This application claims priority of Australian PS0765/02 filed Feb. 26,2002 and PCT/AU02/00939 filed Jul. 13, 2002 and PCT/AU03/00235 filedFeb. 26, 2003, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to fuel cells and is particularlyconcerned with gas separators between adjacent fuel cells, especiallysolid oxide fuel cells.

BACKGROUND OF THE INVENTION

The purpose of a gas separator in a fuel cell assembly is to keep theoxygen containing gas supplied to the cathode side of one fuel cellseparate from the fuel gas supplied to the anode side of an adjacentfuel cell, and to conduct heat generated in the fuel cells away from thefuel cells. The gas separator may also conduct electricity generated inthe fuel cells between or away from the fuel cells. Although it has beenproposed that this function may alternatively be performed by a separatemember between each fuel cell and the gas separator, much developmentwork has been carried out on electrically conductive gas separators.

Sophisticated ceramics for use in fuel cell gas separators have beendeveloped which are electrically conductive, but these suffer from arelatively high fragility, low thermal conductivity and high cost.Special metallic alloys have also been developed, but it has proveddifficult to avoid the various materials of the fuel cell assembly andthe interfaces between them degrading or changing substantially throughthe life of the fuel cell, particularly insofar as their electricalconductivity is concerned, because of the tendency of differentmaterials to chemically interact at the high temperatures that arerequired for different operation of a solid oxide fuel cell. Forexample, most metallic gas separators contain substantial quantities ofthe element chromium, which is used to impart oxidation resistance tothe metal as well as other properties.

It has been found that where chromium is present in more than minutequantities it may combine with oxygen or oxygen plus moisture to formhighly volatile oxide or oxyhydroxide gases under conditions that aretypical of those experienced in operating solid oxide fuel cells. Thesevolatile gases are attracted to the cathode-electrolyte interface wherethey may react to form compounds that are deleterious to the efficiencyof the fuel cell. If these chromium reactions are not eliminated orsubstantially inhibited, the performance of the fuel cell deteriorateswith time to the point where the fuel cell is no longer effective.

Several of these metallic alloys and one proposal for alleviating thisproblem are described in our patent application WO96/28855, in which achromium-containing gas separator is provided with an oxide surfacelayer that reacts with the chromium to form a spinel layer between thesubstrate and the oxide surface layer and thereby tie in the chromium.However, these specialist alloys remain expensive for substantial use infuel cell assemblies and it would be preferable to have a lower costalternative.

Special stainless steels have also been developed that are stable athigh temperature in the atmospheres concerned, but they generallycontain substantial amounts of chromium to provide the desired oxidationresistance, and special coatings or treatments are required to preventthe chromium-based gases escaping from a gas separator formed of thesesteels. Another approach to a heat resistant steel gas separator isdescribed in our patent application WO 99/25890. However, all of theseheat resistant steels are specialist materials and their cost willremain high unless substantial amounts can be produced. Furthermore, thethermal and electrical conductivities of heat resistant steels are lowrelative to many other metals and alloys, for example 22-24 W/m.Kcompared to 40-50 M/m.k for the Siemens-Plansee alloy described inWO96/28855. To compensate for this, the thickness of the steel gasseparator has to be increased, increasing the mass and cost of a fuelcell stack.

In yet another proposal, disclosed in our patent application WO00/76015, we have found that copper-based gas separators may besuccessfully utilized in solid oxide fuel cell assemblies withoutpoisoning the anode. Such a gas separator member comprises a layer ofcopper or copper-based alloy having a layer of oxidation-resistantmaterial on the cathode side.

One of the major difficulties with developing a satisfactory gasseparator is ensuring that its coefficient of thermal expansion (“CTE”)is at least substantially matched to that of the other components of thefuel cell assembly. For example, solid oxide fuel cells comprising anoxide electrolyte with a cathode and an anode on opposed surfacesoperate at temperatures in excess of 700° C., and the alternating gasseparators and fuel cells are generally bonded or otherwise sealed toeach other. Thus, any substantial mismatch in the CTE between the twocomponents can lead to cracking of one or both of them, with resultantleakage of the fuel gas and oxygen-containing gas across the componentor components, and eventually to failure of the fuel cell stack.

A particular difficulty with developing a suitable fuel cell gasseparator is providing a material that provides all four functions ofseparating the fuel gas on one side from the oxygen-containing gas onthe other side, being thermally conductive, having a CTE substantiallymatched to that of the other fuel cell components, and beingelectrically conductive.

In order to meet these requirements, it has been proposed to provide agas separator plate formed principally of a material that may not beelectrically conductive, or not adequately electrically conductive, butthat meets the other requirements, and to provide electricallyconductive feedthroughs through the thickness of the separator. One suchproposal is made in Kendall et al. in Solid Oxide Fuel Cells IV, 1995,pp.229-235, in which the plate is formed of a zirconia material andlanthanum chromite rivets extend through holes in the plate. Anotherproposal for electrically conductive feedthroughs through the thicknessof the separator plate is made in EP 0993059. In this proposal, aceramic gas separator plate, preferably stabilized zirconia, haspassages therethrough that in the preferred embodiment are filled withcathode material from the cathode side and with anode material from theanode side. Alternatively, they may be filled with a single materialcomposition such as doped chromite, silver-palladium or Plansee alloy.

Thus, the feedthrough material is different to that of the principalseparator material and will generally have a higher electricalconductivity. However, as the gas separator is subjected to thermalcycling, this can lead to the disadvantage of the feedthrough materialbecoming loose in the plate material, due to their different CTEs, andto the leakage of gas through the passages in which the feedthroughs areformed.

Additionally in EP 0993059, individual contacts for the feedthroughs of,for example, Ni, Plansee metal or Ag—Pd on the anode side and Ag—Pd orlanthanum strontium manganite on the cathode side, are bonded to therespective electrode by means of a bond layer that overlies the entireelectrode surface. Such a bond layer will tend to inhibit free gas flowthrough the electrode and the individual contacts must be located veryaccurately on the electrodes to overlie the respective feedthrough whenthe fuel cell plates carrying the electrodes and the individual contactsare assembled with the gas separator plates

An alternative proposal published in U.S. Patent Application 20020068677on 6 Jun. 2002 includes a gas separator plate in which the principalplate material is a high silica glass matrix having a metal conductorincorporated therein formed of a material such as silver, Ag—Pd alloy,gold and ferritic stainless steel.

An aim of the present invention is to provide a fuel cell gas separatorthat alleviates the above mentioned disadvantages.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided afuel cell gas separator comprising a first layer which is formed of amaterial that is impermeable to gases, a second layer which is formed ofa material that is impermeable to gases, the first and second layershaving perforations through their thickness which are closed byelectrically conductive plug material, and a third intermediate layerbetween the first and second layers which is electrically conductive andis in electrical contact with the plug material in the perforationsthrough the first and second layers.

By the present invention, the advantage of separating the electricalconductivity of the gas separator from the principal material of theseparator is achieved, and the risk of leakage of gases through theperforations should the plug material become loose is alleviated byproviding the intermediate layer between the first and second layers.Advantageously, the plug material and third intermediate layer provide athermally conductive path for transmission of heat away from the fuelcells on opposite sides.

The materials of the first and second layers of the gas separator arepreferably the same, in order to minimize differences in the CTE of thetwo layers, but this is not essential. The material or materials arepreferably selected with a CTE that substantially matches that of thefuel cell components, but any suitable material may be selected,including electrically conductive materials such as metals and alloys.In a solid oxide fuel cell assembly, in which the electrolyte materialis preferably a zirconia and may be the principal layer that supportsthe electrode layers, the material of the first and second layers of thegas separator is advantageously zirconia. The zirconia of the first andsecond layers may be yttria-stabilized, for example 3 to 10% Y.Alternatively, the zirconia may include other materials while retaininga zirconia-based structure. For example, the zirconia may be a zirconiaalumina having up to 15 wt %, or even up to about 20 wt %, alumina. Forconvenience, all such materials are hereinafter referred to as zirconia.

The thickness of each of the first and second layers of the gasseparator is preferably in the range 20 to 250 μm. While a lesserthickness could be used, the layer becomes difficult to manufacture andit becomes more difficult to ensure that the principal material of thelayer is dense, that is that it is gas tight to the gases in the fuelcell assembly. Greater thicknesses may be used but are unnecessary, andmore preferably the thickness is less than 150 μm in order to minimizethe mass and height of the gas separator plate and therefore of a stackof fuel cell assemblies including the gas separator. Most preferably thethickness of each of the first and second layers is in the range of 50to 100 μm.

The first and second layers may be formed by any suitable means,depending particularly upon their material and the shape of theseparator. A gas separator for use with a planar fuel cell willgenerally be in the form of a plate, that is at least substantiallyplanar, and a planar zirconia first or second layer, for example, may beformed by tape casting the green material and sintering. Suitablemanufacturing methods may be readily identified and do not form part ofthe present invention.

For convenience, the perforations preferably extend perpendicularlythrough the layers. However, this is not essential and it may beadvantageous for the paths of electrically conductive material to beinclined to the perpendicular. In a preferred embodiment, theperforations in the first layer are offset relative to those in thesecond layer so that any gas that is able to leak through one or moreperforations in one of the first and second layers must pass along oracross the third intermediate layer and leak through one or moreperforations in the other of the first and second layers in order toleak through the full thickness of the gas separator.

The perforations through the first and second layers preferably have adiameter or average cross-sectional dimension in the range of 50 to 1000μm. The perforations may be formed during manufacture of the respectivelayer or subsequently, for example by laser cutting. The minimum size ofthe perforations is a function of the difficulty of forming and fillingthem with the electrically conductive plug material. More preferably,the average cross-sectional dimension is in the range 200 to 400 μm, forexample about 300 μm.

The minimum number of perforations is a function of their size, theelectrical conductivity of the plug material and the electrical currentto be passed through the gas separator. If the perforations have anaverage cross-sectional dimension towards the upper end of the preferredrange, they may be fewer in number and more widely spaced. This may beadvantages in alleviating the risk of leakage through the aforementionedpreferred embodiment of the gas separator plate, since the gas may havefurther to travel between the perforations in the first and secondlayers, along or across the third intermediate layer. Preferably, thetotal area of perforations through each of the first and second layersis in the range of 0.1 mm² to 20 mm² per 1000 mm² surface area (on oneside only) of an electrode-contacting zone of the layer, more preferablyin the range 0.2 mm ² to 5 mm² per 1000 mm². In our currently preferredembodiment, there are 19 perforations having an average diameter ofabout 300 μm through each of first and second layers having anelectrode-contacting zone or functional gas separating area of about5400 mm². All of these are sealed with plug material to prevent fuelcell gases passing through the layer.

By the term “electrode-contacting zone” as used throughout thisspecification is meant the portion of each of the first and second gasseparator layers that is opposed to and aligned with the respectiveelectrode of the adjacent fuel cell. Any contact of theelectrode-contacting zone with the adjacent electrode may be indirect,through interposed current collection and/or gas flow control devices.It will be understood therefore that the use of the term“electrode-contacting zone” does not require that zone of the gasseparator layer to directly contact the adjacent electrode.

Advantageously, the electrically conductive plug material in theperforations of each of the first and second layers is the same, and,more conveniently, is the same as the electrically conductive materialof the third intermediate layer, but this is not essential. Any suitableconducting material may be utilized for the plugs and for the thirdintermediate layer including conducting metals, oxides and ceramics. Onepossible conducting oxide is coobaltite. However, conducting metals arepreferred, particularly, for example, Ag, Au, Pt, Ni, alloys containingone or more of these, and other silver-based materials. The preferredmaterial of the third intermediate layer and the preferred plug materialis silver or a silver-based material. The silver or silver-basedmaterial may be metallic silver (commercially pure), a metallic mixturein which Ag is the major component, or a silver alloy.

Particularly if the fuel cell operating temperature will be higher thanabout 900° C. above the melting point of the silver, for example up to1100° C., the silver may be alloyed with any suitable ductile metal ormetals having a sufficiently high melting point. Examples of such metalsare one or more noble metals such as gold, palladium and platinum.Preferably, there will be no less than 50 wt % Ag present in the alloy.An alternative and cheaper material to combine with the Ag is stainlesssteel. The Ag and stainless steel may be mixed as powders and sinteredtogether by firing in the perforations through the first and secondlayers.

The metallic silver, silver mixture or silver alloy electricallyconductive material may be introduced to the perforations by anysuitable method, including screen or stencil printing a slurry of themetal, mixture or alloy in an organic binder into the perforations, orcoating a surface of the first and second layers by, for example,printing, vapour deposition or plating and causing the coated metal,mixture or alloy to enter the perforations.

Most preferably, the electrically conductive material for the plugsand/or the third intermediate layer is a silver-glass composite. Thishas the advantage of separating the desired level of electricalconductivity of the gas separator from the material of the first andsecond layers by the use of silver in the perforations, and alleviatingthe risk of leakage of gases through the gas separator by the use ofclass in the perforations. The glass in the plugs material and/or in thethird intermediate layer may soften at the operating temperature of thefuel cell and, if necessary, can flow with expansion and contraction ofthe first and second layers as the separator is subjected to terminalcycling. The ductility of the silver facilitates this. The silver-glasscomposite may effectively be in the form of pure silver or asilver-based material in a glass matrix.

The silver-glass composite preferably comprises from about 10 to about40 wt % glass, more preferably from 15 to 30 wt % glass. About 10 wt %glass is believed to be the lower limit to provide adequate sealingadvantages in the separator, while at a level above about 40 wt % glassthere may be insufficient silver in the composite to provide the desiredlevel of electrical conductivity. Potentially, the proportions of silverand glass in the composite may be varied to best suit the CTE of thefirst and second layers but the major advantages of the composite lie inthe ability of the material to deform with expansion and contraction ofthe separator and to conduct electricity.

The silver-glass composite may be formed by a variety of suitableprocesses, including mixing glass and silver powders, mixing glasspowder with silver salts, and mixing sol-gel glass precursors and silverpowder or silver salts. Alternatively, for example, the silver or silversalt may be introduced to the glass matrix after the glass particleshave been provided in the principal material of the gas separator, asdescribed hereinafter. The material is then fired. One suitable silversalt is silver nitrate. In a preferred embodiment the glass powder has aparticle size of less than 100 μm, most preferably with an averageparticle size in the range of 13 to 16 μm, and the silver powder has aparticle size less than 45 μm. A suitable binder is for example anorganic screen printing medium or ink. After mixing and application ofthe material, it is fired.

The silver in the composite may be commercially pure, a material mixturein which Ag is the major component or, for example, a silver alloy.

Silver may advantageously be used alone in the glass matrix provided theoperating temperature of the fuel cell is not above about 900° C., forexample in the range 800 to 900° C. There may be some ion exchange ofthe silver at the interface with the glass that may strengthen theAg-glass bond and may spread interface stresses.

Particularly if the fuel cell operating temperature will be higher thanabout 900° C., above the melting point of the silver, for example up to1100° C., the silver may be alloyed with any suitable ductile metal ormetals having a sufficiently high melting point, for example one or morenoble metals such as gold, palladium and platinum. Preferably, therewill be no less than 50 wt % Ag present in the alloy. If the highmelting temperature alloying metal or metals excessively reduces theability of the silver alloy to bond with the glass by ion exchange atthe interface, a lower melting temperature metal such as copper may bealso included.

An alternative and cheaper material to combine with the Ag is stainlesssteel. The Ag and stainless steel may be mixed as powders prior to beingcombined with the glass.

A variety of different glass compositions can be used with the materialof the first and second layers. The glass composition should be stableagainst crystallisation (for example, less than 40% by volumecrystallisation) at the temperatures and cool-down rates at which thefuel cell gas separator will be used. Advantageously, the glasscomposition has a small viscosity change over the intended fuel celloperating range of, for example, 700 to 1100° C., preferably 800 to 900°C. At the maximum intended operating temperature, the viscosity of theglass should not have decreased to the extent that the glass is capableof flowing out of the separator under its own weight.

Preferably, the glass is low (for example, less than 10 wt %) in or freeof fuming components, for example no lead oxide, no cadmium oxide, nozinc oxide, and no or low sodium oxide and boron oxide. The type ofglasses that exhibit a small viscosity change over at least the 100° C.temperature range at the preferred fuel cell operating range of 800° C.to 900° C. are typically high silica glasses, for example in the range55 to 80 wt % SiO₂. Such glasses generally have a relatively low CTE.Preferred and more preferred compositions of such a high silica glass,particularly for use with a zirconia gas separator body, are set out inTable 1.

TABLE 1 Preferred Range More Preferred Range Oxide wt % wt % Na₂O  0-5.5   0-2.0 K₂O  8-14   8-13.5 MgO   0-2.2   0-0.05 CaO 1-3   1-1.6SrO 0-6 0.5-1   BaO 0-8   0-4.4 B₂O₃  6-20  6-20 Al₂O₃ 3-7   3-6.0 SiO₂58-76 6.0-75  ZrO₂  0-10   0-5.0

The composite electrically conductive material may be introduced to theperorations by any suitable means. For example, after the glass powderor particles have been introduced to the perforations of a silver saltor very fine suspension of the silver material, for example as a liquidcoating applied to one or both surfaces of each of the first and secondlayers, may be permitted or caused to be drawn through the glassparticles in the perforations, such as by capillary action.Alternatively, the solution or suspension could be injected in. Morepreferably, a mixture of the glass and silver material powders in abinder is printed, for example by screen or stencil printing, onto oneor both surfaces of each of the first and second layers to at leastpartly fill the perforations. The mixture is then heated to melt theglass and sinter the silver. The molten glass-silver composite thenflows in the perforations to seal them. A suitable heating/firingtemperature is dependent upon the glass composition and the silvermaterial but is preferably in the range 650 to 950° C. for pure silverin a high silica glass matrix for optimum melting of the glass withoutundue evaporation of the silver.

Instead of introducing the plug material to the perforations asdescribed above, more preferably the plug material is introduced in themanner described below with reference to the second aspect of theinvention.

Preferably, the third intermediate layer has a thickness in the range 10to 500 μm, but advantageously the thickness is towards the lower end ofthe range in order to minimize the mass and height of the gas separator.More preferably, therefore, the thickness of the third intermediatelayer is in the range 10 to 100 μm, most preferably less than 60 μm.

The total thickness of the first, second and third layers (i.e.excluding any surface formations on the first and second layers) ispreferably no more than 500 μm. Most preferably the thickness issubstantially less than this in order to minimize the overall height andmass of a full cell stack utilizing the gas separator plates, forexample no more than 200 μm.

The third intermediate layer may be formed by any suitable means, forexample by sputtering, screen printing, tape casting or otherwisecoating onto one of the first and second layers as described above,after which the other of the first and second layers is superposed ontothe third intermediate layer. Alternatively, third intermediate layermaterial may be coated on to both of the first and second layers, withthe first and second layers then being superposed to form theintermediate layer. Either of these coating procedures may be used toalso plug the perforations in one or both of the first and secondlayers, if the material of the intermediate layer is the same as theplug material. Preferably, the third intermediate layer is initiallylaid with a relatively greater thickness than the final thickness and issubsequently compressed between the first and second layers to densifythe third intermediate layer such that it will not permit thetransmission of the gases in the fuel cell assembly. Such pressure maybe used to force the material of the third intermediate layer into theperforations of the first and second layers.

Thus, according to a second aspect of the present invention there isprovided a method of forming a fuel cell gas separator which comprises:

-   -   providing first and second layers of the gas separator, said        first and second layers being formed of material that is        impermeable to gases and having perforations through their        thickness;    -   superposing the first and second layers with a third layer of        electrically conductive material having a first thickness        interposed between the first and second layers;    -   compressing the superposed first, second and third layers under        conditions which cause the electrically conductive material to        flow to produce a gas separator plate in which the third layer        of electrically conductive material has a second thickness less        than the first thickness and said electrically conductive        material has flowed into the perforations in the first and        second layers to plug said perforations.

The method of the second aspect of the invention may be performed at atemperature at which the electrically conductive material flows at thepressure used in the method.

Preferably, the method of the second aspect of the invention includesforming or orienting the first and second layers such that theperforations through said layers are not coincident.

In order to ensure that the fuel cell gas separator does transmitelectricity from the fuel cell on one side to the fuel cell on the otherside, the electrically conductive plug material in the perforations mayextend to the exposed face of the respective one of the first and secondlayers. Alternatively, the plug material may have an electricallyconductive coating on it which extends to the exposed surface and whichmay protect the plug material and/or the interface between the gasseparator and the adjacent electrode. Such a coating will depend uponthe plug material and will also depend upon to which electrode therespective first or second layer is exposed. For example, for a silveror silver-based plug material, a Ni or Pt protective coating may beprovided at the anode side, optionally with an undercoating of Ag, andAg or Ag alloy such as Ag—Sn protective coating may be provided at thecathode side. An Ag—Sn protective coating at the cathode side willparticularly alleviate loss of the aforementioned silver-glass compositeplug material through evaporation or “Nicking” to other nearbycomponents. In particular, the coating may alleviate loss of the glassin the silver-glass composite to the adjacent fuel cell electrode orother porous component by capillary action at the fuel cell operatingtemperature. The coatings also enhance electrical contacts and provide adegree of compliance. Compliance may be provided by the coatings actingto distribute uneven loads due to components of a fuel cell assembly orstack having slightly different heights.

To enhance electrical current flow between the adjacent fuel cell andgas separator, the aforementioned protective coatings advantageouslyextend across the electrode-contacting zones of the separator. Thepreferred coating of nickel, preferably commercially pure nickel, on theanode-facing side may have a thickness in the range of about 10 to 1000μm, preferably 60 to 100 μm. To ensure continued contact of the Ni layerwith the respective one of the first and second layers during extendedthermal cycling of the fuel cell stack, particularly where the materialof the first or second layer is zirconia, a layer of silver preferablycommercially pure Ag, may be disposed on the electrode-contacting zonebetween the coating of nickel and the anode-facing side of the first orsecond layer. Such a layer of silver may have a thickness in the rangeof about 10 to 1000 μm, preferably 20 to 200 μm, and convenientlyprovides enhanced compliance of the overall coating on the anode sidedue to its ductility.

An Ag—Sn alloy coating on the cathode-facing side of the first or secondlayer preferably contains from about 4 to about 20 wt % Sn, and may havea thickness in the range of about 10 to 1000 μm, preferably 100 to 150μm.

The Ag—Sn alloy coating may have a surface layer of SnO₂, formed forexample in the oxidising atmosphere on the cathode-facing side of thegas separator. The SnO₂ surface layer alleviates loss of Ag by“evaporation” at the elevated temperatures of use of the gas separator.To improve the electrical conductivity of the coating on thecathode-facing side, the coating can include up to about 10 wt % ofdopants such as Pd and La. Each of the coating layers may be applied byany suitable means, including screen printing, spin coating, a vapourdeposition process such as magnetron sputtering, slurry coating and tapecasting.

As an alternative to the Ag—Sn coating, a layer of silver may be formedon the cathode-facing side of the separator body. The silver coating onthe cathode side may have a thickness of 10 to 1000 μm, preferably 50 to250 μm.

Alternatively or in addition to the aforementioned coatings, arespective mesh or other current collector may be interposed between thegas separator and the electrodes of the adjacent fuel cells. The mesh orother current collector may define, or partly define, gas passagesthrough which the air or other oxygen-containing gas on the cathode sideof the gas separator and the fuel gas on the anode side of the gasseparator is passed over the adjacent fuel cell electrode.

Alternatively, or in addition, to the aforementioned gas flow passagesdefined or partly defined by a current collector, surface formations maybe provided on the electrode-contacting zones of the gas separator todefine gas flow passages. The surface formations may be in the form ofparallel ridges which may be integrally formed in the material of thefirst and second layers, or may be affixed to the surfaces of the firstand second layers. The surface formations may have any suitable heightto provide for the necessary gas flow, for example up to about 750 μm,preferably about 500 μm high.

Advantageously, in one embodiment, the plug material-filled perforationsare covered by an array of parallel ridges on both sides of theseparator, which extend parallel to the desired direction of the gasflow. The ridges on opposed sides of the gas separator may extendparallel to each other or perpendicularly to each other, depending uponwhether the fuel gas and oxygen-containing gas are to be in co- orcounter-flow, or in transverse—or cross-flow. The ridges may be formedof any suitable material that is electrically conductive andstructurally and chemically stable in the fuel cell operatingenvironment. In one embodiment, the ridges are conveniently bonded tothe aforementioned Ni and silver or Ag—Sn coatings on the separator, orpossibly through the Ni coating to the Ag undercoating if it is present.The ridges on each side of the gas separator are conveniently made ofthe same material as the respective electrode that they contact. Thus,on the cathode side the ridges may be formed of a conductive perovskitesuch as lanthanum strontium manganate, preferably coated with a metallicsilver coating up to about 100 μm, preferably about 50 μm, thick. On theanode side, the ridges may be formed of a nickel-zirconia cermet,preferably with a metallic nickel coating up to about 100 μm, preferablyabout 50 μm, thick.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of a fuel cell gas separator plate in accordancewith the invention will now be described by word of example only withreference to the accompanying drawings in which:

FIG. 1 is a schematic exploded side view of a fuel cell stackincorporating one embodiment of a gas separator plate in accordance withthe invention;

FIG. 2 is a schematic side view of one of the fuel cell gas separatorplates of FIG. 1 during its manufacturing process;

FIG. 3 is a plan view of a fuel cell stack incorporating anotherembodiment of gas separator plates in accordance with the invention;

FIG. 4 is an exploded schematic perspective view looking downwards andillustrating the general orientation of cell plates and gas separatorplates within the stack shown in FIG. 3;

FIG. 5 is a schematic perspective view looking upwards at the cellplates and gas separator plates in the same exploded positions shown inFIG. 4;

FIG. 6 is a perspective view of the topside of one of the cell platesshown in FIG. 4;

FIG. 7 is a cut-away perspective view of the topside of one of the gasseparator, plates shown in FIG. 4;

FIG. 8 is a cut-away underside view of the gas separator plate shown inFIG. 7; and

FIG. 9 is a diagrammatic cross-sectional view through a portion of a gassealed assembly between the plates shown in FIGS. 3 and 4; and

FIG. 10 is an exploded perspective view of another embodiment of gasseparator plate in accordance with the invention, shown with one of twoassociated fuel cell plates.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1 there is shown an array 10 of alternating fuel cells12 and gas separator plates 14 in accordance with the invention. Thefuel cells 12 are planar and comprise a solid oxide electrolyte supportlayer 16 with an anode layer 18 on one side and a cathode layer 20 onthe other side. The electrolyte is preferably a yttria-stabilizedzirconia such as 3Y, 8Y or 10Y. The anode is preferably anickel-zirconia cermet and the cathode is preferably a conductiveperovskite such as lanthanum strontium manganate. Such solid oxide fuelcells are well known and will not be described further.

Each gas separator plate 14 has a three-layer sandwich structure withfirst and second outer layers 22 and 24, and a third intermediate layer26.

The first and second layers are conveniently formed of a zirconia tosubstantially match the CTE of the electrolyte support layer 16 of thefuel cells 12. The zirconia may be yttria-stabilized, but could be, forexample, an alumina-added zirconia with up to 20 wt. % alumina.

The zirconia is not electronically conductive, and each of the first andsecond layers 22 and 24 has a plurality of perforations 28 extendingperpendicularly through its thickness which are filled with anelectrically conductive plug material. The plug material is convenientlya silver-based material such as commercially pure silver, a silver alloyor a composite of silver in glass. In the preferred embodiment, the plugmaterial is a composite of 80 wt % silver in glass. The silver iscommercially pure and the glass has a composition of 0.8-1.2 wt % Na₂O,8.9-9.2 wt % K₂O, <0.1 wt % MgO, 1.4-1.5 wt % CaO, <0.1 wt % SrO,3.1-4.2 wt % BaO, 7.2-10.2 wt % B₂O₃, 6.2-6.6 wt % Al₂O₃, and 68.8-70.4wt % SiO₂.

The perforations 28 in the first and second layers in each gas separatorplate 14 are offset so that no perforation in the first layer iscoincident with a perforation in the second layer. The material of thethird intermediate layer 26 is the same as the plug material in thefirst and second layers, and has a thickness of less than 60 μm.

The perforations have an average cross-sectional dimension of about 300μm, and the plug material 30 seals the perforation to present a totalcross-sectional area of plug material in the range of 0.2 to 5 mm² per1000 mm² of surface area of the electrode-contacting zone on one sideonly of each of the first and second layers.

FIG. 2 illustrates one method of forming the gas separator plate 14. Inthis method, a thicker layer of the material of the third intermediatelayer, for example about 200 μm, is screen printed onto the innersurface of the second layer 24. The screen printing may be performed ator near room temperature. The precursor is a mixture formed bymechanical agitation of powdered glass having a particle size of lessthan 100 μm and an amperage size range of 13 to 16 μm and silver metalpowder having a particle size range of less than 45 μm hinder. Asuitable binder system is a combination of a screen printing inksavailable under the brand name CERDEC and DURAMAX.

As shown in FIG. 2, some of the coated material enters the perforations28 in the second layer during the coating process. The first layer 22 isthen superposed onto the coated material of the third layer on thesecond layer, with the perforations 28 in the first lager offsetrelative to the perforations 28 in the second layer. Sufficient pressureis then applied through the first layer 22, as represented by thedownwards arrows in FIG. 2, to cause the material to flow. This reducesthe precursor material of the third layer to the desired thickness andforces the composite material further into the perforations 28 of thesecond layer 24 as well as into the perforations 28 in the first layer22 to act as the electrically conductive plugs. The gas separator plateis then fired at a temperature of about 850 to 920° C. to melt the glassand sinter the silver into a continuous electrically conductive path inthe glass matrix. At the operating temperature of the fuel cell, theglass in the composite is a viscous fluid and forms a gas barrier, whilethe silver provides the electrical conductivity. At low temperatures, orat shutdown, the molten-viscous glass in the composite returns to asolid/rigid state. Should the composite material become damaged ill thiscondition, once it returns to operating temperature the glass returns toa fluid state and will recover its sealing properties.

A similar forming process may be adopted when the electricallyconductive material comprises silver metal, a silver mixture or silveralloy, with the metal or alloy powder or powders being formed into aslurry and screen printed.

The outer surfaces of the gas separator plate 14 are then coated withconductive layers of an Ag—Sn alloy on the cathode side and nickel metalon the anode side, optionally with a layer of silver between the anodeside and the nickel coating, to protect the electrically conductivepaths. A layer of SnO₂ may overlie the Ag—Sn coating. The ductility ofthe coatings may alleviate stresses arising from uneven loaddistribution in the fuel cell stack due to slightly, different heightsof the components. The coatings may also act to fill the perforations 28from the outside and alleviate wicking of the glass into adjacent porouslayers of the fuel cell stack, so as to ensure that electricallyconductive paths are provided via the perforations and the thirdintermediate layer from one of the outer surfaces to the other outersurface of the gas separator plate 14, as represented by the upwardsarrows in FIG. 1. Additional detail on the application processes for thecoatings is given below.

Referring to FIG. 1 again, one current collector 32 is illustratedschematically between the upper pair of fuel cell 12 and gas separatorplate 14, and this may define gas flow passages between the twostructures. Such gas flow passages are necessary between each pair ofadjacent gas separator plates and fuel cells, but are omitted from FIGS.1 and 2 for convenience only. They are conveniently in the form ofridges on the outer surfaces of the gas separator plates 14, over any ofthe aforementioned coatings as illustrated in and described withreference to FIGS. 3 to 9.

Referring to FIGS. 3, 4 and 5 a solid oxide fuel cell stack assembly 102comprises a stack 103 of alternating fuel cell components, in the formof cell plates 110 and gas separator plates 130 held within a tubularhousing 104. All of the cell plates 110 are identical and all of theseparator plates 130 are identical. As with the plates in FIGS. 1 and 2,typically there might be 20 to 500 of each of these plates in the stack103. Fuel gas and air are supplied at one axial end of the stackassembly and exhaust gases are collected at the other end in aco-current manifolding system. Either end is suitable for the supply andexhaust functions, but the manifold system may alternatively becounter-current. In the described concurrent embodiment, the fuel andair supplies are both at the bottom and exhausts are at the top, but inmany circumstances it is preferred for the fuel to be supplied from thebottom and the air to be supplied from the top in a counter-currentarrangement. Alternatively, all of the gas supplies and exhausts may beat the same end.

Referring to FIGS. 3 to 8, each cell plate 110 has a substantiallycentral, square anode layer on an upper face of the electrolyte-basedcell plate and a substantially central, square cathode layer on a lowerface of the cell plate to form a substantially square fuel cell 112.

The cell plates 110 and separator plates 130 have the same outer shape,which could be described as generally trilobular, or part way between acircle and a triangle. The shape could alternatively be described asgenerally circular with three rounded lobes extending therefrom. Two ofthe lobes 174 and 176 are the same size and the third lobe 172 extendsabout 50% further than the others circumferentially around the peripheryof the plate. At each lobe 172, 174 and 176 a kidney shaped aperture(numbered 114, 116 and 118 in the cell plate and 115, 117 and 119 in theseparator plate, respectively) extends through the plate. The largerlobes 172 carry the larger apertures 114 and 115. A system of rib orridge-shaped seals oil the faces of the plates directs the gas flowswithin the stack. These seals are described hereinafter in more detail,but it will be appreciated that other types of seals may be utilizedincluding gasket seals.

Fuel distribution and exhaust collection manifolds 105 and 106respectively, (see FIG. 3) defined by the three aligned series ofapertures 114, 115, 116 and 117, and 118 and 119 in the fuel cell andgas separator plates and formed by interlocking seal components of theplates 110 and 130, conduct the fuel inlet and exhaust streams past theair side of the plate on the anode side. Air supply and collectionmanifolds 107 and 108 respectively are created by three volumes formedbetween the periphery of the stack 103 and the inside wall of thehousing 104. Manifold 107 is formed essentially between the lobes 174and 176 of the plates, and the two exhaust manifolds 108 are formedessentially between the lobes 172 and 174 and the lobes 172 and 176,respectively, of the plates. Air inlet manifold 107 has an angularextent that is about 50% larger than the each of the two exhaustmanifolds 108, and is opposite the fuel inlet or distribution manifold105. Respective elongate, sliding fibrous seals 109 extend along thestack adjacent the lobes 172, 174 and 176, between the stack 103 and theinside wall of the housing 104 to separate the air supply manifold 107from the two air collection manifolds 108. The fibrous seals may permita degree of leakage between the manifolds 107 and 108, but this is notlikely to be detrimental to the operation of the stack.

The housing 104 is constructed of a suitable heat resistant steel sheetmaterial which lay be lined with a suitable insulating material, and islid into position over the stack 103 after the plates 110 and 130 havebeen assembled together.

In operation of the stack, fuel gas flows up through the longer aperture114 defining an inlet port in each cell plate 110 and (at arrow A)across the face of the fuel cell anode, then divides its flow (arrows Band C) to exit up through exhaust port apertures 117 and 119,respectively, in the adjacent gas separator plate 130. On the oppositeface of the cell plate 110 air, which has passed up the side of thestack 103 through the inlet manifold 107 between the stack and thehousing, flows in (arrow D) from the periphery of the stack 103 andacross the face of the fuel cell cathode, in counter-current to the fuelgas flow across the fuel cell anode before dividing its flow to exit(arrows E and F) from the periphery of the stack 103 and then continuingup through the exhaust manifolds 108 to the top of the stack.

Referring to FIG. 6, the generally planar cell plate 110 used in thestack 103 is shown in greater detail. The square fuel cell 112 on theplate (the anode side is visible) has an electrolyte supported structurewith the electrolyte material extending out to form the main body of theplate 110. The electrolyte is preferably a yttria-stabilized zirconiaand suitable 3Y, 8Y and 10Y materials are known to those in the art. Theanode is preferably a nickel-zirconia cermet and the cathode ispreferably a conductive perovskite such as lanthanum strontiummanganate. The underside of the cell plate 110 and the cathode arevisible in FIG. 5.

A pair of parallel ribs 120 and 121 project from the planar surface 124of the cell plate 110 forming a valley or groove 122 therebetween. Thesurface 124 is the upper, anode surface of the cell plate when the stackis oriented for use. The ribs are formed of zirconia and may beintegrally formed with the main body of the plate or may be formedseparately, for example from a screen printed slurry, and be fired intointegral relationship with the main body. Each rib 120 and 121 forms acontinuous path or closed loop outwardly of the apertures 114, 116 and118 through the cell plate and around the perimeter of the region whichthe fuel gas is permitted to contact. In particular, the closed loopdefined by the ribs 120 and 121 is waisted alongside the anode to directfuel gas from the inlet aperture 114 over the anode.

In all of FIGS. 3 to 9, the thickness of the plates 110 and 130 and theheight of the ribs are shown greatly exaggerated to assist theexplanation of the components. In this embodiment, the fuel cell 112 is2500 mm², the cell plate is 150 μm thick and the ribs are approximately500 μm high, 1 mm wide and approximately 2 mm apart. On the lower,cathode side 154 of each fuel cell plate 110, as shown in FIG. 5, arespective single rib 134 (that corresponds to the ribs 120 and 121 interm of size and how it is formed) extends from the planar surface 154around each of the apertures 114, 116 and 118 through the plate. Each ofthe ribs 134 around the apertures 116 and 118 has an arm 135 thatextends inwardly and towards the aperture 114 (but short thereof)alongside the cathode layer of the fuel cell 112 to assist guidance ofincoming air over the cathode. One of the ribs 134 is also shown in FIG.9 and the use of the rib seals is described with reference to thatFigure.

FIGS. 7 and 8 show the planar gas separator plate 130 in greater detail.In FIG. 7, the surface 133 is the upper, cathode-contacting surface ofthe separator plate 130 when the stack is oriented for use. Respectivepairs of parallel ribs 136 and 137 project from the planar surface 133of the separator plate 130 forming valleys or grooves 138 therebetween.The pairs of parallel ribs 136 and 137 correspond to the ribs 120 and121 in terms of size, spacing and how they are formed, but extend aroundthe apertures 115, 117 and 119 through the plate 130 to cooperate withthe ribs 134 on the cathode-side of the adjacent fuel cell plate 110.The respective ribs 136 associated with the apertures 117 and 119 eachhave a double-walled arm 139 that is closed at its distal end tocooperate with and receive the arm 135 of the corresponding rib 134.

On the lower, anode-contacting side 132 of each gas separator plate 130,a single rib 142 is shown in FIG. 8 and partly in FIG. 7. The rib 142corresponds to the ribs 120 and 121 in terms of size and how it isformed, and forms a continuous path outwardly of the apertures 115, 117and 119 through the plate 130 and around the perimeter of the regionthat the fuel gas is permitted to contact. The outline of the rib 142corresponds to the groove 122 between the ribs 120 and 121 on the anodesurface of the adjacent fuel cell plate 110 and cooperates with thoseribs in forming a seal.

As explained hereinafter, with reference to FIG. 9, glass sealant 140 isused in both of valleys 122 and 138 to form a seal between the ribs.

Each separator plate 130 is manufactured as described with reference toFIGS. 1 and 2 with first and second outer layers formed from a zirconiato substantially match the CTE of the main body of the cell plates 110.This greatly minimizes thermal stresses in the assembly during start-up,operation and shut-down. The zirconia may be yttria-stabilized, butcould be, for example, an alumina-added zirconia with up to 20 wt %alumina, preferably up to 15 wt % alumina. A third, intermediateelectrically conductive layer is disposed between the outer layers in asandwich structure.

The zirconia is not electrically conductive, and the outer layers of theseparator plate 130 have an array of perforations 150 extendingperpendicularly through their thickness that are filled with and extendto the electrically conductive material of the third intermediate layer.These perforations may be formed by laser cutting and occupy a region inthe plate 130 which is directly opposite the region occupied by the fuelcell 112 in plate 110. As in FIGS. 1 and 2, the perforations 150 in thefirst and second layers are offset relative to each other and areconnected via the third intermediate layer. The electrically conductivematerial may be metallic silver (commercially pure) which is plated intothe perforations by standard plating or printing techniques.Alternatively, the electrically conductive material may be a silvermixture, a silver alloy or a silver composite, such as a composite ofsilver, silver mixture or silver alloy in glass. Suitable alloyingelements or materials include gold, palladium and platinum.Alternatively, the silver may be mixed with stainless steel, for exampleas powders prior to sintering in the perforations. In the preferredembodiment, the third intermediate layer is formed with and theperforations are filled with a silver-glass composite of the type and inthe manner described with reference to FIGS. 1 and 2.

The perforations have an average cross-sectional dimension of about 300μm, and the plug material seals the perforations to present a totalcross-sectional area of plug material in the range of 0.2 to 5 mm² per1000 mm² of the electrode-contacting zone (measured on one side only ofthe plate 130). The electrically conductive silver based plug whichfills each perforation is plated with a protective Ni coating on theanode side and an Ag or Ag—Sn coating on the cathode side. The coatingsextend over the entire electrode contacting zone of the plate. Thenickel coating may have an undercoating of Ag to assist the Ni to bondto the separator body and to enhance the compliance of the anode-sidecoating. An Ag—Sn coating may have an SnO₂ surface layer as a result ofbeing oxidized. Such coatings, for example by screen printing of powdermaterials in a binder and subsequent firing, may act to fill theperforations 150 from the outside so as to ensure that electricallyconductive paths are provided via the perforations from one of the outersurfaces to the other outer surface of the gas separator plate. By wayof example only, the nickel coating, the Ag undercoating (if present)and the Ag—Sn coating may have thicknesses in the range of 60-100 μm,20-200 μm and 100-150 μm, respectively. The alternative Ag coating onthe cathode side may have a thickness in the range of 50 to 250 μm.

The coating materials may be applied by any suitable process to achievethe required thickness and consistency, including screen printing apaste of the metal, metals or alloy powder made using a suitable binder,spin coating using a suspension of the metal, metals or alloy powder, aphysical vapour deposition process such as magnetron sputtering, slurrycoating or tape casting.

In a particular embodiment, a nickel coating having a thickness of about80 μm was formed on the anode side of the electrode-contacting zone byscreen printing a paste of Ni powder in a suitable commerciallyavailable organic binder with no Ag undercoating, and then firing the Nilayer. Initially, the Ni layer was oxidized on a first firing. During asubsequent firing in a reducing atmosphere such as hydrogen or fuel gas,the Ni oxide was reduced and the Ni layer actively bonded to theelectrode-contacting zone on the anode side of the gas separator plateto lower the contact resistance, protect the plug material in the seals,and provide a degree of compliance within the stack as a result of theductility of the nickel layer.

On the cathode side in the particular embodiment, an Ag—Sn alloy layerhaving a thickness of about 140 μm was produced on theelectrode-contacting zone by screen printing a paste of the alloy powderin an organic binder. The alloy powder contained 8 to 1.0 wt % Sn in Ag.The screen printing was followed by heating the coating to a temperaturein the range of 500 to 950° C. in all oxidising atmosphere, during whicha Continuous SnO₂ surface layer was formed on the coating. As with thenickel layer, the Ag—Sn alloy layer protects the plug material in theperforations, lowers the contact resistance of the gas separator plateand provides a degree of compliance due to its ductility.

An array of parallel ridges 148 is positioned parallel to the air flowstream in the electrode contacting zone on the cathode side 133 of eachplate 130. These ridges 148 are each aligned over a corresponding row ofperforations 150 and over the Ag—Sn coating. To assist explanation,about half of the ridges 148 have been removed in FIGS. 4 and 7. Theridges 148 perform two major functions. First they provide a conductivepath between the plug material in perforations 150 and Ag—Sn coating andthe fuel cell 112. Second they provide physical support to brace thethin and fragile cell plate as well as means for distributing gas flowsin the narrow spaces between the cell plates and the separator plates.The ridges 148 thus need to be both electrically conductive andstructurally stable. The ridges 148 are approximately 500 μm high andcould be made from a conductive perovskite, such as the LSM material ofthe cathode, optionally with a metallic silver coating about 50 μm thickover the ridges.

On the underside of plate 130 (ie. surface 132 shown in FIG. 8), therows of plugged perforations 150 and the Ni coating are covered by anarray of parallel ridges 162 that are positioned parallel to the fuelgas flow stream. Again, about half the ridges 162 are cut away in FIGS.5 and 8 to assist visualization of the structure. The ridges 162 performas a current collector whereby current is conducted between the plugmaterial in the perforations 150 and the Ni coating and the anode. Theyalso provide physical support for the cell plate and additionallyprovide means for directing and distributing gas flows in the narrowspaces between the cell plates and separator plates. The ridges areapproximately 500 μm high and could be formed from the same material asthe anode, optionally with all overlay (approx 50 μm thick) of nickel.

Referring FIG. 9, a pool of glass sealant 140 is located in the valley138 between the ribs 136 and 137 and is pressed into by rib 134. Eachrib has a tapered profile with oppositely inclined flanks and a distalsurface. A similar arrangement applies between the ribs 120 and 121 andrib 142, but will not be described separately. During manufacture, theglass is introduced as a powder and the stack assembled before the stackis heated to melt the glass in order to form the required seal. Thus, nobinder is required. In operation of the stack the glass sealant 140 isfully molten but highly viscous and is retained in the valley 138 by oneof the following three options not shown in FIG. 9. The glassadvantageously has a composition range of 0-0.7 wt % Li₂O, 0-1.2 wt %Na₂O, 5-15 wt % K₂O, 0-2 wt % MgO, 2-8 wt % CaO, 0-2 wt % SrO, 2-12 wt %BaO, 2-10 wt % B₂O₃, 2-7 wt % Al₂O₃, 50-70 wt % SiO₂ and 0-2 wt % ZrO₂.

In one embodiment option, the distal surface peak 151 of the rib 134contacts the floor 156 of the groove 138 leaving at least one of theflanks 152 of the rib 134 clear of the flanks 158 of the groove andleaving the distal surfaces 160 and 161 of ribs 136 and 137 clear of thebasal surface 154 of the plate 110. In this case the glass sealant 140would be retained by surface tension between the spaced flanks 152 and158.

In a second, and preferred, embodiment option, the distal surfaces 160and 161 contact the basal surface 154 leaving at least one of the flanks152 clear of the flanks 158 and the distal surface 151 clear of thefloor 156. In this case the sealant 140 would be retained between thedistal surface 151 of the rib 134 and the floor 156, with some displacedoutwardly to between the spaced flanks 152 and 158.

In a third embodiment option, both flanks 152 would engage correspondingflanks 158 leaving the distal surfaces 160 and 161 clear of the basalsurface 154 and the distal surface 151 clear of the floor 156. In thiscase the sealant 140 would fill the volume between the distal surface151 and the floor 156.

Referring now to FIG. 10, there is shown (in exploded manner) a fuelcell plate 210 superposed over a gas separator plate 212. In use, theplates 210 and 212 are in at least substantially face to face contactand there would be a stack of alternating fuel cell plates 210 and gasseparator plates 212.

The plates 210 and 212 are seen in perspective view from above with acathode layer 214 visible on an electrolyte layer 216 on the fuel cellplate 210. An anode layer (not visible) corresponding to the cathodelayer 214 is provided on the underside (in the drawing) of the fuel cellplate.

The fuel cell and gas separator plates 210 and 212 are generallycircular and are internally manifolded with a fuel inlet opening 218, afuel outlet opening 220, air inlet openings 222 and air outlet openings224, which respectively align when the plates are stacked. A gasket-typeseal 226 and 228, respectively, is provided on the upper face (in thedrawing) of each of the fuel cell and gas separator plates 210 and 212.The gasket-type seals 226 and 228 are conveniently formed of a glasscomposition or a glass composite.

The seal 226 has air inlet ports 230 associated with the air inletpassages and air outlet ports 232 associated with the air outletpassages 224 to permit air to flow across the cathode layer 214 betweenthe cathode and the adjacent gas separator plates (not shown). The seal226 extends wholly around the fuel inlet passage 218 and outlet passage220 to prevent fuel flowing over the cathode side of the fuel cell plate210.

Correspondingly, the seal 228 on the gas separator plate 212 extendswholly around the air inlet passages 222 and the air outlet passages224, but only around the exterior of the fuel inlet passage 218 andoutlet passage 220 so that fuel gas can flow from the fuel inlet passe218, across the anode, between the fuel cell plate 210 and adjacent gasseparator plate 212, before exiling through the fuel outlet passage 220.

Means (not shown) is provided to distribute the reactant gas across therespective electrode and to provide support for all of the plates 210and 212 in a fuel cell stack. Such means may be in the form of surfaceformations oil the gas separator plate 212, for example as describedwith reference to FIGS. 3 to 9, or on the fuel cell plate 210.Alternatively, the gas may be distributed by a separate member betweenthe plates, such as a mesh or corrugated structure, that may also act asa current collector.

As before, the cathode material is preferably a conductive perovskitesuch as lanthanum strontium manganate that is porous, and the anode ispreferably formed of a porous nickel-zirconia cermet.

The electrolyte layer 216 is preferably a yttria-stabilized zirconiasuch as 3Y, 8Y or 10Y and extends beyond the electrode layers to definethe internally manifolded fuel and air inlet and outlet passagestherethrough, to support the seal 226 and to provide a contact surfacefor the seal 228 on the gas separator plate 212.

The gas separator plate has a similar profile to the fuel cell plate 210and has a sandwich structure of zirconia outer layers to substantiallymatch the CTE of the electrolyte layer 216 of the fuel cell and anelectrically conductive third intermediate layer. The zirconia of thegas separator plate 212 may be yttria-stabilized, but could be, forexample, an alumina-added zirconia with up to 20 wt. % alumina.

Since the zirconia of the outer layers is non-electrically conductiveand one of the functions of the gas separator plate 212 is to transmitelectrical current from one fuel cell to the next through the stack,offset electrically-conductive passages 234 are provided through thethickness of a planar central portion or electrode-contacting zone 240of the outer layers corresponding in shape and size to the adjacentelectrode to connect with the third intermediate layers. The passages234 comprise substantially perpendicular perforations through the outerlayers of the plate 212 containing a silver or silver based materialprovided, along with the third intermediate layer, as described withreference to FIGS. 1 to 9. Although the passages 234 through the gasseparator plate 212 are illustrated as visible, they would be coveredwith an electrically-conductive coating across the central portion 236on each side, also as previously described herein.

Whilst the above description includes the preferred embodiments of theinvention, it is to be understood that many variations, alterations,modifications and/or additions may be introduced into the constructionsand arrangements of parts previously described without departing fromthe essential features or the spirit or ambit of the invention.

It will be also understood that where the word “comprise”, andvariations such as “comprises” and “comprising”, are used in thisspecification, unless the context requires otherwise such use isintended to imply the inclusion of a stated feature or features but isnot to be taken as excluding the presence of other feature or features.

The reference to any prior art in this specification is not, and shouldnot be taken as, an acknowledgment or any form of suggestion that suchprior art forms part of the common general knowledge.

1. A fuel cell gas separator comprising a first layer which is formed ofa material that is impermeable to gases, a second layer which is formedof a material that is impermeable to gases, the first and second layershaving perforations through their thickness which are closed byelectrically conductive plug material, and a third intermediate layerbetween the first and second layers which is electrically conductive andis in electrical contact with the plug material in the perforationsthrough the first and second layers, wherein said third intermediatelayer is comprised of an electrically conductive material selected fromthe group consisting of electrically conductive metal alloys, metals,metal oxides, ceramics, and glass composites.
 2. The gas separatoraccording to claim 1, wherein the materials of the first and secondlayers are the same.
 3. The gas separator according to claim 1, whereinthe material of each of the first and second layers is zirconia.
 4. Thegas separator according to claim 3, wherein the zirconia isyttria-stabilised.
 5. The gas separator according to claim 3, whereinthe zirconia contains up to about 20 wt. % alumina.
 6. The gas separatoraccording to claim 1, wherein the thickness of each of the first andsecond layers is in the range 20 to 250 μm.
 7. The gas separatoraccording claim 1, wherein the thickness of the third intermediate layeris in the range 10 to 100 μm.
 8. The gas separator according to claim 1,wherein the electrically conductive material of the third intermediatelayer is selected from the group consisting of cobaltite, Ag, Au, Pt,Ni, alloys containing one or more of said metals, and other silver-basedmaterials.
 9. The gas separator according to claim 1, wherein thematerial of the third intermediate layer is the same as the electricallyconductive plug material.
 10. The gas separator according to claim 1,wherein the perforations extend perpendicularly through the thickness ofthe first and second layers.
 11. The gas separator according to claim 1,wherein the perforations in the first layer are offset relative to theperforations in the second layer.
 12. The gas separator according toclaim 1, wherein each perforation has an average cross-sectionaldimension in the range of 50 to 1000 μm.
 13. The gas separator accordingto claim 1, wherein the total area of the perforations through each ofthe first and second layers is in the range of 0.1 to 20 mm² per 1000mm² surface area of an electrode-contacting zone of said layer.
 14. Thegas separator according to claim 1, wherein the electrically conductiveplug material is selected from the group consisting of cobaltite, Ag,Au, Pt, Ni, alloys containing one or more of said metals, and othersilver-based materials.
 15. The gas separator according to claim 14,wherein the electrically conductive plug material is selected from thegroup consisting of metallic silver, a metallic mixture in which Ag isthe major component, a silver alloy and a silver-glass composite. 16.The gas separator according to claim 15, wherein the electricallyconductive plug material is silver alloyed or mixed with any one or moreof gold, palladium, platinum or stainless steel.
 17. The gas separatoraccording to claim 15, wherein the electrically conductive plug materialis a silver-glass composite containing from about 10 to about 40 wt %glass.
 18. The gas separator according to claim 17, wherein thesilver-glass composite contains from about 15 to 30 wt % glass.
 19. Thegas separator according to claim 15, wherein the electrically conductiveplug material is a silver-glass composite in which the silver isselected from the group consisting of commercially pure silver and asilver alloy or mixture.
 20. The gas separator according to claim 19,wherein the silver is alloyed or mixed with any one or more of gold,palladium, platinum or stainless steel.
 21. The gas separator accordingto claim 15, wherein the electrically conductive plug material is asilver-glass composite in which the glass is stable againstcrystallization.
 22. The gas separator according to claim 15, whereinthe electrically conductive plug material is a silver-glass composite inwhich the glass is a high silica glass.
 23. The gas separator accordingto claim 22, wherein the composition of the glass is 0-5.5 wt % Na₂O,8-14 wt % K₂O, 0-2.2 wt % MgO, 1-3 wt % CaO, 0-6 wt % SrO, 0-8 wt % BaO,6-20 wt % B₂ 0 ₃, 3-7 wt % Al₂O₃, 58-76 wt % SiO₂ and 0-10 wt % ZrO₂.24. The A gas separator according to claim 23, wherein the compositionof the glass is 0-2.0 wt % Na₂O, 8-13.5 wt % K₂O, 0-0.05 wt % MgO, 1-1.6wt % CaO, 0.5-1 wt % SrO, 0-4.4 wt % BaO, 6-20 wt % B₂O₃, 3-6.0 wt % Al₂_(O) ₃, 60-75 wt % SiO₂ and 0-5.0 wt % ZrO₂.
 25. The gas separatoraccording to claim 1, wherein a respective electrically conductivecoating is provided on the electrically conductive plug material at theelectrode-facing side of each of the first and second layers.
 26. Thegas separator according to claim 25, wherein each of said coatingsextends over a respective electrode-contacting zone of each of the firstand second layers.
 27. The gas separator according to claim 25, whereinthe electrically conductive coating on a cathode-facing side is of Ag orAg alloy.
 28. The gas separator according to claim 27, wherein thecoating on the cathode-facing side is Ag—Sn alloy that contains fromabout 4 to about 20 wt % Sn.
 29. The gas separator according to claim27, wherein the coating on the cathode-facing side is Ag—Sn alloy thatincludes up to 10 wt % of dopants to improve the electrical conductivityof said coating.
 30. The gas separator according to claim 27, whereinthe coating on the cathode-facing side is Ag—Sn alloy and has athickness in the range of 10 to 1000 μm.
 31. The gas separator accordingto claim 27, wherein the coating on the cathode-facing side is Ag—Snalloy having a surface layer of SnO₂.
 32. The gas separator according toclaim 27, wherein the coating on the cathode-facing side is ofcommercially pure silver and has a thickness in the range of 50 to 250μm.
 33. The gas separator according to claim 25, wherein theelectrically conductive coating on an anode-facing side is of nickel.34. The gas separator according to claim 33, wherein the nickel coatingon the anode-facing side is commercially pure.
 35. The gas separatoraccording to claim 33, wherein the layer of nickel on the anode-facingside has a thickness in the range of 10 to 1000 μm.
 36. The gasseparator according to claim 33, wherein a layer of silver is disposedon the electrode-contacting zone between the coating of nickel, and theanode-facing side of the respective first or second layer.
 37. The gasseparator according to claim 36, wherein the layer of silver comprisescommercially pure silver.
 38. The gas separator according to claim 36,wherein the layer of silver has a thickness in the range of 10 to 100μm.
 39. The gas separator according to claim 1, wherein surfaceformations defining gas flow passages therebetween are provided on anelectrode-facing side of each of the first and second layers, saidsurface formations being electrically conductive and overlying theperforations containing the electrically conductive plug material. 40.The gas separator according to claim 39, wherein the surface formationson an anode-facing side are formed of solid oxide fuel cell anodematerial and the surface formations on a cathode-facing side are formedof solid oxide fuel cell cathode material, said surface formations beingbonded to the first and second layers or to any electrically conductivecoating thereon.
 41. The gas separator according to claim 39, wherein arespective electrically conductive coating is provided over the surfaceformations on the anode-facing side and on the cathode-facing side. 42.The gas separator according to claim 41, wherein the coating on thesurface formations on the cathode-facing side is of metallic silver. 43.The gas separator according to claim 41, wherein the coating on thesurface formations on the anode-facing side is of nickel.
 44. A methodof forming a fuel cell gas separator which comprises: providing firstand second layers of the gas separator, said first and second layersbeing formed of material that is impermeable to gases and havingperforations through their thickness; superposing the first and secondlayers with a third layer of electrically conductive material having afirst thickness interposed between the first and second layers;compressing the superposed first, second and third layers underconditions which cause the electrically conductive material to flow toproduce a gas separator in which the third layer of electricallyconductive material has a second thickness less than the first thicknessand said electrically conductive material has flowed into theperforations in the first and second layers to plug said perforations.45. The method according to claim 44, wherein the first and secondlayers are formed or oriented such that the perforations through saidfirst and second layers are not coincident.
 46. A fuel cell gasseparator comprising a first layer which is formed of a material that isimpermeable to gases, a second layer which is formed of a material thatis impermeable to gases, the first and second layers having perforationsthrough their thickness which are closed by electrically conductive plugmaterial, and a third intermediate layer between the first and secondlayers which is electrically conductive and is in electrical contactwith the plug material in the perforations through the first and secondlayers, wherein the perforations in the first layer are offset relativeto the perforations in the second layer.
 47. A fuel cell gas separatorcomprising a first layer which is formed of a material that isimpermeable to gases, a second layer which is formed of a material thatis impermeable to gases, the first and second layers having perforationsthrough their thickness which are closed by electrically conductive plugmaterial, and a third intermediate layer between the first and secondlayers which is electrically conductive and is in electrical contactwith the plug material in the perforations through the first and secondlayers, wherein surface formations defining gas flow passagestherebetween are provided on an electrode-facing side of each of thefirst and second layers, said surface formations being electricallyconductive and overlying the perforations containing the electricallyconductive plug material.
 48. The gas separator according to claim 47,wherein the surface formations on an anode-facing side are formed ofsolid oxide fuel cell anode material and the surface formations on acathode-facing side are formed of solid oxide fuel cell cathodematerial, said surface formations being bonded to the first and secondlayers or to any electrically conductive coating thereon.
 49. The gasseparator according to claim 47, wherein a respective electricallyconductive coating is provided over the surface formations on theanode-facing side and on the cathode-facing side.
 50. The gas separatoraccording to claim 48, wherein the coating on the surface formations onthe cathode-facing side is of metallic silver.
 51. The gas separatoraccording to claim 48, wherein the coating on the surface formations onthe anode-facing side is of nickel.